CORRELATED RATES OF MOLECULAR AND MORPHOLOGICAL EVOLUTION

Since Zuckerkandl and Pauling (1962, 1965) proposed the molecular cloc k, many studies seem to have supported their prediction that rates of molecular and morphological evolution generally will be decoupled. Mos t of these studies were aimed at taxa in which rates of morphological evolution were thought to vary greatly a priori. For the current surve y eight diverse taxa were systematically chosen from published studies without regard to prior expectations about rates. Two approaches show ed that rates of molecular and morphological evolution may usually be coupled. First, correlations in the total number of changes accumulate d in terminal taxa suggest that some mechanism alters the rates of bot h morphological and molecular evolution in concert. Second, node-densi ty effects were removed statistically, and average corrected base-to-t ip totals were compared among sister clades. Across all taxa 50 of 72 of these corrected contrasts support the hypothesis that rates of mole cular and morphological evolution are correlated; this finding is high ly significant by a binomial test. Furthermore, there were positive co rrelations between inferred molecular and morphological branch lengths in seven of eight cases, which is also significant. These branch leng th correlations are consistent with the rate correlations, and suggest that amounts of molecular and morphological evolution often are corre lated also. This study supports the assumptions of several phylogeneti c methods, and highlights a need for new inquiries into many aspects o f both molecular and morphological evolution.